1. Field of the Invention
This invention affords a method and an apparatus for reducing the nitrogen oxide (NO.sub.x) content of stack gases produced by the burning of fossil fuels, such as coal, gas, or oil, in furnaces, especially industrial furnaces serving steam boilers.
2. Statement of the Related Art
In furnaces, particularly of the large industrial type, an excess of NO.sub.x in the stack (exhaust) gas is an acute environmental problem. In the Federal Republic of Germany, an ordinance related to such furnaces demands the limitation of NO.sub.x to about 200 mg/Nm.sup.3 when coal is the fuel and about 100 mg/Nm.sup.3 when gas is the fuel. Similar federal, state, and/or local ordinances exist in other countries. This has resulted in various attempts at compliance.
The reduction of the NO.sub.x content can basically be accomplished by direct modification of the burner or its related apparatus, or by treating the stack gases, for example by catalytic conversion.
It is known that by means of measures related to control of the firing, i.e., with the aid of "low" NO.sub.x burners, NO.sub.x values of about 350 mg/Nm.sup.3 can be achieved, wherein up to 30% of the stack gases produced are returned to the combustion chamber. In this known technology the lowering of the NO.sub.x content is at the expense of a considerable reduction in efficiency due to a distinctly elevated energy expenditure. This is especially the case since larger quantities of stack flow through the furnace, so that increased pressure losses occur and thus a considerable increase in electric current consumption for the blower is necessitated. It has also been found that the effect achieved with flame cooling through the return of up to 30% of the stack gas is not always sufficient to produce the desired reduction in NO.sub.x, completely independently of the above-described disadvantages and various construction problems.
It is also known to retrofit the heating surface in existing furnaces, causing a change in the temperature profile which is undesireable.